1. Field of the Invention
This invention relates to selective spectral output metal halide arc discharge lamps having long life and lumen maintenance. More particularly, this invention relates to selective spectral output metal halide vapor arc lamps for reprographic and photographic processes emitting in the blue, green and red bands wherein the arc tube contains a fill comprising mercury, zinc, indium, lithium, thallium, at least one halogen and a rare earth metal.
2. Background of the Disclosure
Lamps intended for general lighting are designed to achieve the highest visible light radiation efficiency possible together with high color rendition at a specified color temperature. In most cases, this has resulted in solving problems to provide sufficient red radiation in order to achieve a good color rendition of the white light. In such lamps, the electrical characteristics are essentially those of a mercury discharge. However, there are other applications for electric lamps wherein emission scattered throughout the visible spectrum is undesirable. For instance, in reprographic applications for making colored copies, radiation concentrated in the three primary colors, blue, green and red is desired. The three primary colors can be achieved from light sources emitting continuously throughout the visible spectrum by means of filters. In this type of application the light beams are provided either from three separate light sources or by splitting the beam from a single white light source by means of optical filters. Such filters are used to eliminate from the light path everything except the desired primary color, and the three primary colors may then be recombined into a single beam. Such systems are prohibitively expensive as well as inefficient. Similarly, in some photochemical applications high energy emission in specific regions or bands is required in order to achieve a desired chemical reaction, and emission in other bands must be suppressed because it may inhibit the desired reaction and even produce undesirable side reactions.
The principles of color reproduction processes utilizing the three primary colors are well known. In such processes it is important that the light source employed emit radiation in the three primary color spectrums, blue, green and red at wavelengths which will be efficient in producing the desired reaction in the dyes and/or other chemical reagents used. In most color reprographic systems, the dyes, etc., which react with blue light are relatively insensitive to the light radiation in the blue color range. Also, blue light radiation is more readily absorbed by most media which results in low transmission. Consequently, lamps employed with such processes should emit a relatively high level of blue radiation in order to efficiently and effectively produce the desired chemical reaction and concomitant color change in the paper, emulsion, slide, phosphor, liquid crystal or other substrate.
Projection television systems also require light emission in the three primary colors, blue, green and red. The three primary colors containing the desired image or signal are separately projected on a screen wherein the colors combine to produce a desired light image. For color projection processes the primary objectives are good color reproduction and high screen brightness after passing through a medium in which the color information is contained (i.e., liquid crystals, slides, screens), with the lowest possible amount of power dissipation in the light radiation.
U.S. Pat. Nos. 3,840,767 and 3,876,895 describe selective spectral output metal halide vapor arc discharge lamps having light emissions concentrated in the blue, green and red energy bands wherein the relative emission characteristics or energy levels in the three bands are approximately 1:2:2, respectively and wherein little or no blue radiation is emitted at a wavelength of about 450 nm. Both of these lamps contain a fill comprising a mixture of halides of zinc, lithium and thallium, with the lamp of the '767 patent additionally containing a halide of gallium.